1. Field of the Invention
The invention relates to an appliance for extracting secondary air from an axial compressor, a peripheral slot being used for the extraction.
2. Discussion of Background
In compressors, secondary air is extracted to provide cooling air for the turbine. A certain total pressure is necessary, depending on the cooling duty. A peripheral slot is usually employed for extracting the secondary air. The extracted air passes through the extraction slot into the extraction plenum and from there via the extraction tube into the cooling system of the turbine. Unfortunately, a large part of the dynamic pressure is lost during the extraction.
The criteria for optimizing the extraction slot are very contradictory because, on the one hand, the flow in the extraction slot must be optimized but, on the other, the flow in the bladed annular space of the compressor must not be disturbed. This problem is particularly severe where the extraction slot is also used as the blow-off system when the compressor is being run up and run down.
Various forms of such connections between the main duct and the blow-off system have therefore been evolved, such as slots of different shapes, hollow guide vanes and openings in the platforms at the roots of the guide vanes.
If the extraction slots most frequently used are considered, it is found that although a part of the axial component of the flow velocity can be used--depending on the geometry of the slot (particularly the opening angle of the extraction slot and the angle of inclination of the slot relative to the center line of the compressor)--the peripheral component is almost completely dissipated. In modern compressors, however, the energy content of the peripheral component is, in fact, very large so that this leads to significant losses of total pressure. The pressure losses are, inter alia, directly proportional to the square of the peripheral component of the flow velocity in the compressor.